This is a 3-part study to assess the safety, tolerability, efficacy, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of MK-8226 in participants with moderate to severe atopic dermatitis. Part 1 (multiple rising dose study) objectives were to find the maximum tolerated dose (MTD) of MK-8226 and to assess safety and PK. Part 2 objectives were to determine safety, PK, and preliminary efficacy. Part 3 objectives were to further define safety and PK, and explore MK-8226 PK/PD to model the optimal dose range for future studies. The study was terminated early due to business reasons on 08 May 2014; final results from an analysis for Part 1 (efficacy, PK, safety, immunogenicity) and Part 2 (safety, immunogenicity) are summarized.